The invention relates to method and apparatus for the preparation of solid from fluid in general, and more particularly to the removal of slag from a coal-gasification reactor.
Typically, in a coal-gasification plant where the slag evolving in the process falls in the furnace, or in the waste heat boiler associated thereto, the slag collects into a water bath provided at the base of the furnace, or of the associated waste heat boiler. While the slag in free-fall is to be removed at the bottom of the reactor, e.g. of the furnace or of the associated waste heat boiler, a lock hopper is connected beneath the reactor to collect the accumulated slag. The purpose of the water bath at the bottom of the furnace (in the absence of a waste heat boiler), or at the bottom of a waste heat boiler when it exists, is to provide a seal maintaining the internal temperature and pressure in the reactor while the slag is removed. The slag hopper operates as a lock between an upper and a lower door. The upper door admits the slag through the water bath when it is opened and the lower door is closed. Slag recovery is when the lower door is opened and the upper door is closed. A successful operation depends, though, upon the slag sinking right through the water bath onto the upper door of the lock hopper.
Combustion is the most important factor in a coal-gasification process. When combustion is incomplete, there remains in the water bath a floatable slag which does not separate, e.g., which does not sink to the bottom. Therefore, the operation of the reactor has to be interrupted periodically in order that the floatable slag can be removed. In such case, removal has to be done manually which is quite disadvantageous in itself. Such interruption of the reactor is also a drawback for a continuous chemical process.
An object of the present invention is to combine continuous operation of a coal-gasification reactor with slag removal by making such removal possible without it being manual.
The invention stems from the concept of preventing floatability of the slag. According to the invention, an additive is used in such a way that the slag particles will stick to each other. As a result, slag particles of floatable weight and particles of non-floatable weight will unite to form larger particles, whereby sinking of the slag is ensured. Heavier slag particles, because they are attached to the floatable ones, tend to entrain the lighter slag particles to the bottom.
According to still another feature of the invention, the falling slag is at least in part recycled by mixing with the reactor charging materials. This provides several advantages. In particular, an improved performance of the reactor, an extended combustion of the slag particles which were incompletely burnt and reuse of the added oil are obtained.
Preferably, the slag is fed back as charging material after passing through a classifier in the form of a grader, or a separator. As a result, the slag particles which are fully or mostly burnt can be separated. This is achieved by imparting to the particles, after sinking through the water bath from the boiler or its associated waste heat boiler, a horizontal motion right above a series of funnels or successive surfaces of the classifier. The slag particles of larger weight fall with a relatively large vertical velocity into the closest funnel, or are lodged onto the closest surface in the direction of travel, while the lighter slag particles fall at more or less remote distances into different other funnels, or settle onto different other surfaces, in accordance with their respective weights.